1. Field of the Invention
The present invention generally relates to dielectric films having a polymer layer, such as a short chain polymer layer, including Si—C bonds, and in particular to low dielectric constant films of the type, for example, that are used as insulating layers on semiconductor wafers, and methods of producing such insulating layers.
2. Description of the Related Art
The desire to produce semiconductor wafers with high device densities and high speeds has led to a search for low dielectric constant (low k value) films for use as insulating layers in semiconductor devices. Such a dielectric film can be either spun on or deposited using, for example, a chemical vapor deposition process. The dielectric film is then usually heated to harden it. Recent work has been particularly directed to films consisting of polymer layers including Si—C bonds and the Applicants' International Patent Application WO 98/08249 describes, by way of example, such a process for depositing such a film.
The reduction in dielectric constant in such films seems to derive from a decreased density of the film, possibly due to a disruption of the lattice by the carbon atoms present in the film.
The dielectric material is formed or deposited upon the wafer as a short chain polymer that is further polymerized and hardened by heating to between 400° C. and 500° C., typically in a nitrogen atmosphere of between 30 mTorr and 20 Torr, and more generally in the absence of oxygen at any convenient pressure.
It has become apparent, however, that at least some dielectric films of this sort are susceptible to cracking, particularly at the wafer edge. Typically such cracking begins to occur at a nominal film thickness of about 5000-6000 Å. The cause seems in part to be due to a dramatic increase in film thickness within 1 mm of the wafer edge. The cracking appears only to occur following exposure to atmosphere and aggressively travels in towards the center over a period of about 24 hours. The rate of cracking can be delayed by keeping the wafers in vacuum, but eventually they do crack when exposed to atmosphere.
It has also become apparent that the dielectric constant (k value) of some dielectric films formed in this manner increases significantly from its initial value after a period of time. For example, experiments on two samples of dielectric films formed in this manner exhibited initial k values of approximately 2.84 and 2.88, but after 48 hours the k values of the samples had increased to approximately 2.92 and 2.99 respectively.
Accordingly, it would be advantageous to provide a semiconductor device having a dielectric film with a low dielectric constant (k value). It would also be advantageous to provide such a device including a low k value dielectric film that is less susceptible to cracking, and methods of producing such dielectric films. Other and further objects and advantages will appear hereinafter.